Reciprocating internal combustion engine

ABSTRACT

A highly-efficient, yet simply constructed internal combustion engine includes an intake cylinder to accommodate intake and pre-compression of an oxidizing agent, a combustion cylinder to accommodate a further compression of the oxidizing agent, an injection and ignition of fuel, and a partial expansion of combustion gases produced by the ignition of fuel; and an exhaust cylinder to accommodate a further expansion of the combustion gases and subsequent exhausting of the further expanded combustion gases. A reciprocating piston is inside each of the intake, combustion and exhaust cylinders and a crankshaft is coupled to the reciprocating pistons. A first transfer passage facilitates flow of the pre-compressed oxidizing agent from the intake cylinder to the combustion cylinder and a second transfer passage facilitates flow of the partially-expanded combustion gases from the combustion cylinder to the exhaust cylinder.

FIELD OF THE INVENTION

This disclosure relates to a reciprocating internal combustion engineand, more particularly, relates to a high-efficiency, reciprocating,internal combustion engine.

BACKGROUND

A reciprocating internal combustion engine is a heat engine that usesone or more reciprocating pistons to convert the energy of a combustingfuel into a rotating motion.

In a typical reciprocating internal combustion engine, the expansion ofthe high-temperature and high-pressure gases produced by the combustionof fuel inside a cylinder applies force to drive a piston inside thecylinder. This force moves the piston over a distance therebytransforming the chemical energy of the fuel into useful mechanicalenergy.

SUMMARY OF THE INVENTION

In one aspect, a highly-efficient, yet simply constructed internalcombustion engine includes an intake cylinder to accommodate intake andpre-compression of an oxidizing agent, a combustion cylinder toaccommodate a further compression of the oxidizing agent, an injectionand ignition of fuel, and a partial expansion of combustion gasesproduced by the ignition of fuel; and an exhaust cylinder to accommodatea further expansion of the combustion gases and subsequent exhausting ofthe further expanded combustion gases. A reciprocating piston is insideeach of the intake, combustion and exhaust cylinders and a crankshaft iscoupled to the reciprocating pistons. A first transfer passagefacilitates flow of the pre-compressed oxidizing agent from the intakecylinder to the combustion cylinder and a second transfer passagefacilitates flow of the partially-expanded combustion gases from thecombustion cylinder to the exhaust cylinder.

In another aspect, a method includes: pre-compressing an oxidizing agentwith a reciprocating piston inside an intake cylinder of an internalcombustion engine; further compressing the pre-compressed oxidizingagent, injecting and igniting fuel into the further compressed oxidizingagent, and accommodating a partial expansion of resulting combustiongases to drive a reciprocating piston inside a combustion cylinder ofthe internal combustion engine; and enabling the combustion gases tofurther expand inside an exhaust cylinder of the internal combustionengine and expelling the further expanded combustion gases out of theexhaust cylinder with a reciprocating piston inside the exhaustcylinder. The reciprocating pistons inside the intake, combustion andexhaust cylinders are coupled to a crankshaft of the internal combustionengine.

In some implementations, the engine employs the Brayton cycle spreadacross three separate cylinders.

In some implementations, one or more of the following advantages arepresent.

For example, a highly-efficient engine that has a relatively simpledesign and construction may be provided. In addition, the engine mayhave a compact size relative to conventional compression-ignitionengines. Moreover, the engine may have the ability to burn natural gasby utilizing a high overall compression ratio that is not achievable inconventional engines.

Other features and advantages will be apparent from the description anddrawings, and from the claims.

Relative terms, such as “lower,” “upper,” “horizontal,” “vertical,”,“above”, “below”, “up”, “down”, “top” and “bottom” as well as derivativethereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should beconstrued to apply only to certain orientations (e.g., a particularorientation being described or shown in a drawing being discussed).These relative terms are used for convenience only and do not requirethat the apparatus be constructed or operated in a particularorientation.

The dead center in an engine is the position of a piston in which it isfarthest from, or nearest to, the crankshaft. The former is known as topdead center (TDC) and the latter is known as bottom dead center (BDC).These phrases, top dead center and bottom dead center, are usedthroughout the present disclosure and should be construed broadly inmanner consistent with their respective meanings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are partial cross-sectional views showing one implementationof a reciprocating internal combustion engine at various stages of theengine's operation. Like reference numerals in the figures refer to likeelements.

DETAILED DESCRIPTION

The engine 100 shown in FIGS. 1-3 has two power-generating sections 102a and 102 b, each of which is configured to deliver mechanical energy toa common crankshaft 104. Power-generating section 102 a includes threein-line cylinders that are on a first side of the common crankshaft 104and power-generating section 102 b includes three in-line cylinders thatare on a second side of the common crankshaft 104.

As discussed in detail herein., the three cylinders in eachpower-generating section 102 a, 102 b cooperate to accommodate the stepsrequired to conduct one complete engine operating cycle. In generalterms, these steps include intake, compression, combustion, expansionand exhaust. More particularly, as discussed herein, the steps requiredto conduct one complete engine operating cycle (i.e., intake,compression, combustion, expansion and exhaust) are spread out among thethree cylinders in each power-generating section 102 a, 102 b.

In each power-generating section 102 a, 102 b, the three cylindersinclude: an intake cylinder 108, a combustion cylinder 110 and anexhaust cylinder 112. A first transfer passage 114 extends between theintake cylinder 108 and the combustion cylinder 110. A second transferpassage 16 extends between the combustion cylinder 110 and the exhaustcylinder 112.

An intake valve 118 is provided to regulate the flow of fluid (e.g.,air) into the intake cylinder 108. A first transfer valve 120 isprovided to regulate the flow of fluid (e.g., air) through the firsttransfer passage 114 from the intake cylinder 108 to the combustioncylinder 110. A second transfer valve 122 is provided to regulate theflow of fluid (e.g., combustion gases) through the second transferpassage 116 from the combustion cylinder 110 to the exhaust cylinder112. An exhaust valve 124 is provided to regulate the flow of fluid(e.g., exhaust gases) out of the exhaust cylinder 112.

In the illustrated implementation, each valve 118-124 is poppet thatconsists of a hole at the top of one of the cylinders, and a taperedplug on the end of a shaft (also known as a valve stem). The portion ofthe hole where the plug mates to it is called the valve seat. In atypical implementation, the shaft would guide the plug portion bysliding through a valve guide (not shown).

According to the illustrated implementation, in each power-generatingsection 102 a, 102 b, the intake valve 118 is at the top of the intakecylinder 108, the first and second transfer valves 120, 122 are at thetop of the combustion cylinder 110 and the exhaust valve 124 is at thetop of the exhaust cylinder 112.

In a typical implementation, the valves are controlled by a valveactuator 125. More particularly, the valve actuator is configured tocontrol (e.g., open and close) the intake valve, the first transfervalve, the second transfer valve and the exhaust valve at appropriatetimes in the engine's operating cycle to support appropriate engineoperations.

There is a reciprocating piston inside each cylinder. More particularly,there is a reciprocating intake piston 126 inside each intake cylinder108; there is a reciprocating combustion piston 128 inside eachcombustion cylinder 110; and there is a reciprocating exhaust piston 130inside each exhaust cylinder 112. The reciprocating pistons are arrangedsuch that each piston reciprocates inside one of the intake, combustionand exhaust cylinders in a direction that is substantially perpendicularto an axis of the crankshaft. Moreover, each reciprocating piston in aparticular one of the power-generating sections 102 a, 102 b issubstantially parallel to the other reciprocating pistons in itspower-generating section. This type of arrangement is not required, ofcourse. Instead of being parallel, the reciprocating pistons in theparticular power-generating sections 102 a, 102 b can be arrangedradially around the crankshaft, for instance. Other types ofarrangements are possible as well.

In the illustrated implementation, the three cylinders 108-112 in eachpower-generating section 102 a, 102 b have different bore sizes. Forexample, in each power-generating section 102 a, 102 b, the combustioncylinder 110 has the smallest bore size, the exhaust cylinder 112 hasthe largest bore size, and the intake cylinder 108 has a bore size thatis between the bore size of the combustion cylinder 110 and the boresize of the exhaust cylinder 112. Each piston has a diameter thatcorresponds to the bore size of its associated cylinder.

In certain implementations, the ratios of intake piston diameter tocombustion piston diameter to exhaust piston diameter to piston strokeis: 2.15:1:3.37:1.18. Of course, each of these dimensions can vary(e.g., approximately +/−% 15, or more) relative to one or more of theother specified dimensions. How they vary may depend, for example, onthe specific desired engine operational characteristics of theparticular engine.

Each piston 126-130 is coupled to the common crankshaft 104 via arespective connecting rod 132 and connecting rod bearings 134. Thecommon crankshaft 104 is configured to translate the reciprocating,linear motion of the pistons into a rotational motion and vice versa. Ineach power-generating section 102 a, 102 b, there is a fuel injectionmechanism arranged to inject fuel into combustion cylinder 110 at anappropriate point in the engine's operating cycle. In the illustratedimplementation, the fuel injection mechanisms are fuel injectors 134.Notably, the intake cylinders 108 and the exhaust cylinders 112 do nothave any fuel injectors or other fuel injection mechanisms. This isbecause, in each power-generating section 102 a, 102 b, fuel is directlyinjected only into the combustion cylinder 110, not into the intake 108or exhaust 112 cylinders.

In some implementations, fuel may be injected into the first transferpassage between the intake cylinder and the combustion cylinder. Thismay be done instead of or in addition to injecting fuel directly intothe combustion cylinder. This dual-injection technique may be useful,for example, in applications where two different fuels are used. In someimplementations, it may be desirable to inject fuel into the intakecylinder. This, too, may be done instead of or in addition to injectingfuel directly into the combustion cylinder and/or the first transferpassage.

The illustrated engine is a compression-ignition engine (i.e., theengine uses the heat of compression to initiate ignition for burningfuel that has been injected into the engine. However, in someimplementation, there may be a different source of ignition (e.g., aspark plug) configured to ignite the fuel that is injected into thecombustion cylinder 110. Notably, in engines that utilizespark-ignition, the intake cylinders 108 and the exhaust cylinders 112would not have any source of ignition, such as a spark plug. This isbecause, in each power-generating section 102 a, 102 b of a similar,spark-ignition, type of engine, fuel would be ignited only in thecombustion cylinder 110, not in the intake 108 or exhaust 112 cylinders.

A discussion of how the illustrated engine 100 operates follows. Thisdiscussion focuses primarily on power-generating section 102 a. Itshould be understood, however, that power-generating section 102 b hassimilar physical and operational characteristics as power-generatingsection 102 a and operates in similar and coordinated manner withpower-generating section 102 a to cooperatively deliver useablemechanical energy to crankshaft 104.

In general, and as discussed in further detail below, the intakecylinder 108 accommodates intake and pre-compression of an oxidizingagent (e.g., air); the combustion cylinder 110 accommodates furthercompression of the oxidizing agent, fuel injection and ignition of theinjected fuel and a partial expansion of the resulting combustion gases;and the exhaust cylinder 112 accommodates a further expansion of thecombustion gases and ultimately exhausting the expanded combustion gasesfrom power-generating section 102 a.

In FIG. 1, oxidizing agent (e.g., is being drawn into the intakecylinder 108, fuel combustion has just begun or is about to begin in thecombustion cylinder 110 and expanded combustion gases are being expelledfrom the exhaust cylinder 112.

The intake valve 118 is in an open position and the reciprocating pistoninside the intake cylinder 108 is moving in a downward direction fromtop dead center to bottom dead center thereby drawing air into theintake cylinder 108. In some implementations, the intake valve 118remains open for most, if not all, of the time that the reciprocatingpiston inside the intake cylinder 108 is moving from top dead center tobottom dead center.

The exhaust valve 124 is in an open position and the reciprocatingpiston inside the exhaust cylinder 112 is moving in an upward directionfrom bottom dead center to top dead center thereby expelling expandedcombustion gases from the exhaust cylinder 122 In some implementations,the exhaust valve 124 remains open for most, if not all, of the timethat the reciprocating piston inside the exhaust cylinder 112 is movingfrom bottom dead center to top dead center.

Both transfer valves 120, 122 are in a closed position and thereciprocating piston inside the combustion cylinder 110 is atapproximately top dead center. In a typical implementation, this wouldbe just after the fuel has been injected (by fuel injector 136) andignited (e.g., by heat of compression). The resulting combustion gasesinside the combustion cylinder 110 are just starting to expand in thecombustion cylinder 110 and drive the reciprocating piston inside thecombustion cylinder 110 in a downward direction from top dead center (asshown in FIG. 1) toward bottom dead center. In some implementations, thetransfer valves 120, 122 remain closed for most, if not all, of the timethat the reciprocating piston inside the combustion cylinder 110 ismoving from top dead center to bottom dead center.

The crankshaft 104, which is coupled to the reciprocating pistons insidethe intake cylinder, combustion cylinder and exhaust cylinder, istranslating the linear motion of the reciprocating piston in thecombustion cylinder 110 into a rotational motion and, at the timedepicted in FIG. 1, driving the reciprocating pistons in the intakecylinder 108 and exhaust cylinder 112.

In FIG. 2, the illustrated engine components are configured as theywould be approximately 90 degrees after their configuration shown inFIG. 1.

The intake valve 118 and the first transfer valve 120 are in a closedposition and the reciprocating piston inside the intake cylinder 108 hasjust passed bottom dead center and started moving in an upward directionfrom bottom dead center to top dead center thereby compressing thecharge of air that previously was drawn into the intake cylinder 108.

Since the charge of air being compressed in the intake cylinder 108 inFIG. 2 is further compressed in the combustion cylinder 110 at asubsequent point in time, the compression that occurs in the intakecylinder 108 is referred to in some places herein as a pre-compression.

In some implementations, the intake valve 118 remains closed for most,if not all, of the time that the reciprocating piston inside the intakecylinder 108 is moving from bottom dead center to top dead center.Additionally, in some implementations, the first transfer valve 120remains closed for part of the time that the reciprocating piston insidethe intake cylinder 108 is moving from bottom dead center to top deadcenter.

The reciprocating piston inside the combustion cylinder 110 isapproximately at the center of its expansion stroke and is moving in adownward direction from top dead center to bottom dead center.

The second transfer valve 122 between the combustion cylinder 110 andthe exhaust cylinder 112 has just opened. At this point, thereciprocating piston inside the exhaust cylinder 112 is at or very neartop dead center and the volume of space inside the exhaust cylinder 112(i.e., between the top of the reciprocating piston inside the exhaustcylinder 112 and the cylinder head) is quite small. This advantageouslyminimizes blow down losses between the combustion cylinder 110 and theexhaust cylinder 112. In some implementations, the second transfer valve122 will close when the reciprocating piston inside the combustioncylinder reaches close to bottom dead center. This means that there isgenerally some retained combustion gas. Just as the reciprocating pistoninside the combustion cylinder reverses direction, the first transfervalve between the intake and combustion cylinders opens and the intakepiston pushes a fresh charge of air in.

The exhaust valve 124 is in a closed position and the exhaust piston isjust beginning or is about to begin its expansion stroke, moving fromtop dead center to bottom dead center. In some implementations, theexhaust valve 124 remains closed for most, if not all, of the exhaustpiston's expansion stroke, moving from top dead center to bottom deadcenter. In some implementations, the exhaust valve 124 opens when thereciprocating piston inside the exhaust cylinder is close to, at or justpast bottom dead center at the end of its expansion stroke.

With the engine configured as shown in FIG. 2, the combustion gases canexpand to drive the reciprocating piston inside the combustion cylinder110 and to drive the reciprocating piston inside the exhaust cylinder112. The crankshaft 104, which is coupled to the reciprocating pistonsinside the intake cylinder, combustion cylinder and exhaust cylinder, istranslating the linear motion of the reciprocating pistons inside thecombustion cylinder 110 and inside the exhaust cylinder 112 into arotational motion and driving the reciprocating piston inside the intakecylinder 108.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the present disclosure. For example, therelative bore sizes of the intake, combustion and exhaust cylinders maydiffer from what has been disclosed herein. In some implementations, thebore sizes of these three cylinders may be substantially the same as oneanother.

The specific timing of various engine operations (e.g., valve timing,fuel injection timing, etc.) may vary.

The intake, first and second transfer, and exhaust valves may be of adifferent style, size and/or arrangement than what is disclosed herein.For example, the valves may be sleeve valves instead of poppet valves.There may be two, three, four, five or more valves per cylinder.

In addition, the engine may include any number of cylinders as long asthat number is a multiple of three (3). So, for example, the engine mayinclude three cylinders, six cylinders, nine cylinders, twelvecylinders, fifteen cylinders, eighteen cylinders, etc. Each set of threecylinders would include an intake, combustion and exhaust cylinder.

The engine can be arranged with an in-line engine design, (e.g., astraight engine with all of the cylinders placed in a single row), aU-engine design (e.g., two separate straight engines with crankshaftslinked by a central gear), a V-engine design (e.g., two banks ofcylinders at an angle, most commonly 60 or 90 degrees), a flat enginedesign (e.g., two banks of cylinders directly opposite each other oneither side of the crankshaft), and any other type of engine design.

The relative order of cylinders (i.e., intake, combustion and exhaust)may differ from what has otherwise been disclosed herein. So, forexample, the intake cylinder 108 may be physically positioned betweenthe combustion cylinder 110 and the exhaust cylinder 112.

The valve actuator 125 may include a camshaft that revolves at somefixed speed relative to the crankshaft speed (e.g., half-speed). Thecamshaft may be driven in any convenient manner, including for example,by chain, gear or toothed belt driven from the crankshaft, and can belocated, for example, in the engine's crankcase or in the cylinder head.Valves could also be driven electrically or hydraulically. Valve timingcould vary depending on engine operating conditions.

The physical arrangement of the transfer passages may differ. Moreover,the physical positions of the transfer valves in connection with thosetransfer passages may differ as well. For example, in someimplementations, the first transfer valve 120 may be at the beginning ofthe first transfer passage 114 in the cylinder head of the intakecylinder 108. Likewise, in some implementations, the second transfervalve may be at the end of the second transfer passage 116 in thecylinder head of the exhaust cylinder 112.

The various components disclosed can have a variety of shapes and sizes.

The techniques, components and systems disclosed herein can be adaptedfor use in connection with a variety of different engine stylesincluding, for example, engines that run on diesel fuel or other heavyfuels, engines that run on gasoline or alcohols and engines with orwithout spark ignition.

Engines implementing the techniques disclosed herein can be used inconnection with a wide variety of applications including, for example,aircraft auxiliary power units, alternative light vehicle engines,marine engines, on-highway truck engines, military unmanned aerialvehicles, tactical vehicle engines, stationary power generation, andaircraft engines.

The engine can be adapted to include any number of optional accessories(e.g., turbochargers, superchargers, etc.) that might be used in aconventional engine to enhance efficiency.

Other implementations are within the scope of the claims.

What is claimed is:
 1. An internal combustion engine comprising: anintake cylinder to accommodate intake and pre-compression of anoxidizing agent; a combustion cylinder to accommodate a furthercompression of the oxidizing agent, an injection and ignition of fuel,and a partial expansion of combustion gases produced by the ignition offuel; and an exhaust cylinder to accommodate a further expansion of thecombustion gases and subsequent exhausting of the further expandedcombustion gases; a reciprocating piston inside each of the intake,combustion and exhaust cylinders; a crankshaft coupled to thereciprocating pistons; a first transfer passage to facilitate flow ofthe pre-compressed oxidizing agent from the intake cylinder to thecombustion cylinder; and a second transfer passage to facilitate flow ofthe partially-expanded combustion gases from the combustion cylinder tothe exhaust cylinder.
 2. The internal combustion engine of claim 1,wherein the combustion cylinder has a smaller bore than the firstcylinder.
 3. The internal combustion engine of claim 1, wherein theexhaust cylinder has a larger bore than the intake cylinder.
 4. Theinternal combustion engine of claim 1, further comprising a fuelinjector configured to inject the fuel into the combustion cylinder. 5.The internal combustion engine of claim 4, wherein there are no fuelinjectors configured to inject fuel into the intake cylinder or theexhaust cylinder.
 6. The internal combustion engine of claim 1, furthercomprising an ignition source to ignite the fuel in the combustioncylinder.
 7. The internal combustion engine of claim 1, furthercomprising: an intake valve operable to regulate a flow of the oxidizingagent into the intake cylinder; a first transfer valve operable toregulate a flow of the pre-compressed oxidizing agent through the firsttransfer passage from the intake cylinder to the combustion cylinder; asecond transfer valve operable to regulate a flow of thepartially-expanded combustion gases through the second transfer passagefrom the combustion cylinder to the exhaust cylinder; and an exhaustvalve operable to regulate a flow of the further expanded combustiongases out of the exhaust cylinder.
 8. The internal combustion engine ofclaim 7, further comprising a valve actuator operable operably coupledto the intake valve, the first transfer valve, the second transfer valveand the exhaust valve.
 9. The internal combustion engine of claim 8,wherein the valve actuator is operable to: open the intake valve so thatthe oxidizing agent can be drawn into the intake cylinder during atleast part of the time that the reciprocating piston inside the intakecylinder is moving between top dead center and bottom dead center; closethe intake valve to allow the oxidizing agent inside the intake cylinderto be pre-compressed by the reciprocating piston inside the intakecylinder moving between bottom dead center and top dead center; open thefirst transfer valve after the reciprocating piston inside the intakecylinder has pre-compressed the oxidizing agent to allow thepre-compressed oxidizing agent to flow through the first transferpassage from the intake cylinder to the combustion cylinder; close thefirst transfer valve to allow the pre-compressed oxidizing agent to befurther compressed by the reciprocating piston inside the combustioncylinder moving from bottom dead center to top dead center; open thesecond transfer valve after the after fuel combustion has occurredinside the combustion cylinder to allow the partially-expandedcombustion gases to flow through the second transfer passage from thecombustion cylinder to the exhaust cylinder; and close the secondtransfer valve after at least some of the partially-expanded combustiongases have flowed through the second transfer passage from thecombustion cylinder to the exhaust cylinder; and open the exhaust valveafter the partially-expanded combustion gases have further expanded inthe exhaust cylinder to allow the further expanded combustion gases toflow out of the exhaust cylinder while the reciprocating piston insidethe exhaust cylinder is moving from bottom dead center to top deadcenter.
 10. The internal combustion engine of claim 1, wherein thecrankshaft is configured to rotate about an axis and each pistonreciprocates inside one of the intake, combustion and exhaust cylindersin a direction that is substantially perpendicular to the axis of thecrankshaft.
 11. A method comprising: pre-compressing an oxidizing agentwith a reciprocating piston inside an intake cylinder of an internalcombustion engine; further compressing the pre-compressed oxidizingagent, injecting and igniting fuel into the further compressed oxidizingagent, and accommodating a partial expansion of resulting combustiongases to drive a reciprocating piston inside a combustion cylinder ofthe internal combustion engine; and enabling the combustion gases tofurther expand inside an exhaust cylinder of the internal combustionengine and expelling the further expanded combustion gases out of theexhaust cylinder with a reciprocating piston inside the exhaustcylinder, wherein the reciprocating pistons inside the intake,combustion and exhaust cylinders are coupled to a crankshaft of theinternal combustion engine.
 12. The method of claim 11, furthercomprising: opening an intake valve to allow the oxidizing agent to bedrawn into the intake cylinder while the reciprocating piston inside thefirst cylinder is moving between top dead center and bottom dead center.13. The method of claim 12, further comprising: closing the intake valveto allow the oxidizing agent inside the intake cylinder to bepre-compressed by the reciprocating piston inside the first cylindermoving between bottom dead center and top dead center.
 14. The method ofclaim 13, further comprising: opening a first transfer valve after thereciprocating piston inside the first cylinder has pre-compressed theoxidizing agent to allow the pre-compressed oxidizing agent to flow fromthe intake cylinder to the combustion cylinder.
 15. The method of claim14, further comprising: closing the first transfer valve to allow thepre-compressed oxidizing agent to be further compressed by thereciprocating piston inside the combustion cylinder moving from bottomdead center to top dead center.
 16. The method of claim 15, furthercomprising: injecting the fuel into the combustion cylinder afterfurther compressing the oxidizing agent.
 17. The method of claim 16,further comprising igniting the injected fuel in the combustioncylinder.
 18. The method of claim 17, further comprising: opening asecond transfer valve after the after fuel combustion has occurredinside the combustion cylinder to allow the partially-expandedcombustion gases to flow from the combustion cylinder to the exhaustcylinder.
 19. The method of claim 18, further comprising: closing thesecond transfer valve after at least some of the partially-expandedcombustion gases have flowed from the combustion cylinder to the exhaustcylinder.
 20. The method of claim 19, further comprising: opening anexhaust valve after the partially-expanded combustion gases have furtherexpanded in the exhaust cylinder to allow the further expandedcombustion gases to flow out of the exhaust cylinder while thereciprocating piston inside the exhaust cylinder is moving from bottomdead center to top dead center.
 21. The method of claim 10, wherein thecombustion cylinder has a smaller bore than the intake cylinder, andwherein the exhaust cylinder has a larger bore than the intake cylinder.